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REPORT 985-NATIONAL ADVISORY COMMITTEE FOR AERONAiUTICS

APPARATUS _Airplane equipment.-For the calibration tests the air-plane should be equipped with the following instruments: apitot-static installation, a recording normal accelerometer,an airspeed recorder, a recording altimeter, a chronometrictimer, and a radio to communicate the timing signals to theground equipment. The airspeed recorder is used to recordthe impact pressure, or the difference between total andstatic pressures, measured by the pitot-static installation.The recording altimeter is used to record the static pressuremeasured by the static-pressure source. A measure of free-stream temperature by means of a calibrated thermometeris also very desirable but not absolutely necessary. The air-speed recorder and the recording altimeter should be theonly instruments connected to the static-pressure source andshould be located as near to it as possible in order to minimizethe pressure lag of the recording system. The magnitude ofthe pressure lag may be determined by methods describedin reference 1. If the lag is appreciable, corrections must bemade to the measured static pressure.Ground equipment,-The required ground equipmentconsists of a radar unit, a phototheodolite, and a chronograph.The radar unit is directed on a target through the use of thephototheodolite. The radar unit is equipped within a motion-picture camera to photograph the radar scope, a targetcamera, and an elevation-scale camera., The scope photo-graphs give the slant range and the target photographs, thecorrection to _the elevation scales. The_three cameras andthe airplane equipment are synchronized by means of achronograph that records timing impulses from the groundcameras and the airplane timer.CALIBRATION PROCEDURE AND ACCURACYThe calibration procedure -consists essentially in surveyingthe atmospheric pressure over the desired range of altitudeand then flying the airplane to be calibrated through thisregion and recording the static pressure measured by thestatic-pressure source,. Th _.static-pressure error of thepitot-static installation is the difference between atmosphericpressure and the static pressure measured by the static-pressure source.The survey of atmospheric pressure may be made with theairplane being calibrated or with another airplane similarlyinstrumented. In either case the airplane is tracked withthe radar-phototheodolite unit while measurements _of staticpressure are being made with the static-pressure installationat airplane speeds for which a calibration is available or maybe obtained by other techniques such as described in refer-ence 1. The atmospheric pressure so determined is in errorby the static-pressure error of the static-pressure source andmust be corrected for it. Because of the variation of atmaos-pheric pressure with time, the survey should be repeatedfrequently (for instance, between calibrating maneuvers ifnecessary) when the calibration tests are expected to take along time. Variations in sea-level atmospheric pressure

with time of as much as 0.3..inch of water per hour arepossible under some conditions.

When the airplane is also equipped with a thermometerfor determining free-stream temperature, measurements oftemperature are made simultaneously with the measurementsof static pressure during the surveys. From the variationof free-stream temperature with altitude, a fairing of thestatic-pressure data with altitude may be obtained since

P=v Ae R

where _, is arbitrarily selected for the best fairing of thedata.In the calibration maneuvers the airplane should be flownthrough the region surveyed (slant range, elevation, andazimuth) or as near to it as possible. For maneuvers inwhich the airplane cannot be restricted to the region surveyedas a result of the high speed of the airplane, corrections maybe necessary to the data obtained in the survey and in themaneuver for the following: refraction of light, curvatureof the earth, error in alinement of the plhototheodolito refer-ence plane with the horizon, and the variation of atmosphericpressure at a given altitude with distance along the earth'ssurface. The error in altitude due to refraction of light, asdetermined from equations derived in the appendix and thenotation in figure 1, is presented . in figure 2 and tihe errordue to the earth's curvature is given in figure 3. A check ofthe sea-level atmospheric pressure as measured at U. S.weather stations I0 to 30 miles apart indicated that on dayswith surface winds of 20 to 25 miles per hour the horizontalpressure gradients may be as much as 0.05 inch of water permile along the earth's surface. At an altitude of 10,000 feet,weather maps indicated horizontal pressure gradients asmuch as 0.02 inch of water per mile.An SCR-584 radar unit with phototheodolite and cameras(scope, target, and elevation scales) has an estimated accur-acy of about -45 feet in slant. range andl -0.2 nil in eleva-tion for a single observation. For a series of observationswhere the data may be faired, the accuracy, of course, isimproved. The error in altitude due to tie error in slantrange and elevation angle is shown in figure 4. Consistenterrors in the tracking equipment should have no effect onthe accuracy of the calibration, provided that these errorsoccur throughout the calibration procedure.An NACA recording altimeter has a random error of about X percent of full-scale value for a single observation. Thiserror corresponds to errors of -I and =0.2 inch of waterfor altimeters built to measure pressures including sea-levelpressure and to measure pressures at and above an altitudeof 40,000 feet, respectively. The combined maximum prob-able error in a single observation due to errors in measuringstatic pressure and in determining altitude (+45 ft or 0.2in. of water at 40,000 ft) is -1 1 inch of water for the altim-eter with a range including sea-level pressure and 0.3 inchof water for the altimeter with a range for altitudes aboveabout 40,000 feet. Since in the calibration procedure a seriesof observations is made over a range of altitude, the fairingof the data reduces the magnitude of the error.

Citing and Sharing

Reference the current page of this Report.

Zalovcik, John A.A radar method of calibrating airspeed installations on airplanes in maneuvers at high altitudes and at transonic and supersonic speeds,
report,
January 1, 1950;
(digital.library.unt.edu/ark:/67531/metadc60324/m1/2/:
accessed December 13, 2017),
University of North Texas Libraries, Digital Library, digital.library.unt.edu;
crediting UNT Libraries Government Documents Department.